Conventionally, in order to make it possible for a home or an office to receive and transmit high-speed broad band information like ultra high speed data, FTTH (Fiber to the home) i.e. an access line optical fiber cable extended from a telephone company is dropped down to a subscriber""s home such as a building, a house of the general public via an optical fiber cable core wire, and an optical fiber drop cable has been used as a suitable means for wiring. In short, the optical fiber drop cable (an outdoor cable) is a cable used when dropping an optical fiber down from a utility pole into the home.
As a related optical fiber drop cable known is as shown in FIG. 1. The optical fiber drop cable 101 comprises an optical element portion 109 and a support wire portion 113, and an optical fiber tape core wire 103 for example of 0.25 mm diameter and steel wires 105 as tension bodies disposed on both sides of the optical fiber tape core wire for example of 0.4 mm diameter are covered with a thermoplastic resin 107 such as a PVC, a flame-retarded polyolefin to form the optical element portion, and a steel wire 111 as tension body for example of 1.2 mm diameter is covered with the thermoplastic resin 107 such as a PVC, a flame-retarded polyolefin to form the support wire portion. These portions are paralleled one another and are continuously or intermittently fixed together at a thin neck portion.
When a cable is anchored on a utility pole or a building/a house of the general public, it is anchored by means of the support wire portion 113 as a messenger wire after separating the support wire portion 113 from the optical element portion 109 utilizing the thin neck portion.
Taking out the optical fiber tape core wire 103 by tearing off the optical element portion 109 right and left utilizing notch portions 117 which is provided at the central part of the optical element portion, one end of the optical fiber tape core wire is connected to some other fiber within a closure disposed on the utility pole and the other end is connected to a connection box or an OE converter disposed in or out of the house.
1. Flexible Plastic Material
The conventional optical fiber drop cable 101 described above is liable to an accident by lightning strike, for example, burning of indoor equipments. Therefore, the accident may be avoided either by cutting off the conventional optical fiber drop cable 101 within the connection box disposed on the walls of the building and the house of general public in order not to drop the optical fiber drop cable directly into the house, or by cutting off only the steel wire 105 in the optical element portion 109 using a specific tool in order to drop the cable without the steel wire into the house. Besides, the support wire portion 113 has little problem because it will be cut off when the cable is anchored on the utility pole or under eaves of the house of the general public.
According to the present invention, an optical fiber drop cable, which protects an accident by lightning strike and also has low cost, and a manufacturing method thereof can be provided.
According to the first technical aspect of the present invention, an optical fiber drop cable 1 comprising: an optical element portion 9 where an optical fiber core wire 3 and at least one pair of first tension bodies 5, 5 disposed parallel on both sides of the optical fiber core wire are covered with a cable sheath; and a cable support wire portion 15 where a second tension body 11 is covered with a sheath 13, wherein both of the portions are fixed together continuously or intermittently, and said first tension bodies are composed of a plastic material. The tensile elastic modulus of said plastic material is preferably between 10000-50000 N/mm2. Further, a cross section of said first tension bodies is preferably noncircular.
According to the second technical aspect of the present invention, a manufacturing method of the optical element portion 9 of the optical fiber drop cable 1, wherein said optical element portion is fixed to the cable support portion 15 along the primary axis of said cable, and said optical element portion includes first tension bodies 5, 5 comprising a flexible plastic material 37 and an optical fiber core wire 3, 29 all of which are covered with a cable sheath 7 consisting of a resin 39, comprises the steps of: (i) running said optical fiber core wire 29 in the prescribed direction; (ii) extruding said flexible plastic material 37 synchronizing with the running of said optical fiber core wire; and (iii) extruding said resin 39 synchronizing with the running of said optical fiber core wire 29, said flexible plastic material contacting said resin before the plastic material solidifies.
2. Molding of an Electrically Nonconductive Material
Generally, with regard to the optical fiber drop cable 101 having weak adhesion between the tension bodies 105 and the cable sheath 107, there have been such problems that troubles like increase in optical transmission loss or breakdown of the optical fiber core wire 103 might occur when bending, squeezing, etc. were applied, most likely when thermal hysteresis such as heat cycle were additionally applied.
Thus, the adhesion (drawing force) between the tension bodies 105 within the optical element portion 109 and the cable sheath 107 is a necessary item for the characteristics of the optical fiber drop cable. However, when the optical fiber drop cable 101 is manufactured by extrusion molding, simple plain extrusion molding of the electrically nonconductive material such as glass fiber, aramid fiber, FRP being used for the tension bodies 105 within the optical element portion 109 together with the support wire 111, the optical fiber core wire 103, etc. cannot provide suitable adhesion. Therefore, conventionally, an adhesive layer is interposed between the tension bodies 105 and the cable sheath 107 so as to increase adhesion described above, resulting in high cost because of increased manufacturing processes.
According to the present invention, the adhesion between the tension bodies within the optical element portion and the cable sheath is enhanced without interposing the adhesive layer. Moreover, the optical fiber drop cable can be provided with increased anti-torsion characteristics and ability to protect the increase in optical transmission loss.
According to the third technical aspect of the present invention, an optical drop cable 201 comprising: an optical element portion 207 having an optical fiber core wire 205 and a pair of first tension bodies 213, 213, each of said first tension bodies being disposed substantially parallel along the primary axis of said cable on both sides of said optical fiber core wire intervening between them, said pair of the first tension bodies and said optical fiber core wire are covered with a cable sheath 203; and a cable support wire portion 211, including a second tension body 217 covered with a sheath 219, said optical element portion 207 and said cable support wire portion 211 being fixed together along the primary axis of said optical fiber drop cable 201, wherein said first tension bodies are composed of an electrically nonconductive material and a circumference thereof is formed into a rugged shape.